1. Field of the Invention
The invention relates to the axial turbine of a turbocharger.
2. Discussion of Background
The use of exhaust-gas turbochargers for increasing the output of internal combustion engines is widespread nowadays. Here, the exhaust gases of the internal combustion engine are admitted to the exhaust-gas turbine of the turbocharger and their kinetic energy is used to draw in and compress air for the internal combustion engine. As a function of the actual operating conditions and the composition of the fuels used to drive the internal combustion engine, contamination of the moving blades and of the nozzle ring in the exhaust-gas turbine occurs sooner or later, the nozzle ring being affected to a substantially greater extent. In heavy-oil operation, a contamination layer, the hardness of which depends on the working principle of the internal combustion engine, forms on the nozzle ring. In general, such contamination deposits in the region of the nozzle ring lead to a poorer turbine efficiency and consequently to a reduction in the output of the internal combustion engine. In addition, an increase in the exhaust-gas temperatures in the combustion space occurs, as a result of which both the internal combustion engine and the turbocharger may be thermally overstressed. In the internal combustion engine, in particular damage to or even destruction of the valves may occur.
If a contamination layer is deposited on the nozzle ring and the turbine blades of a turbocharger connected to a four-stroke internal combustion engine, an increase in the pressures and in the rotational speed of the turbocharger can be expected. Consequently, components of both the internal combustion engine and the turbocharger are subjected to higher thermal and mechanical stress, a factor which may likewise lead to the destruction of the relevant components. If the contamination layer is distributed irregularly at the periphery of the moving blades of the turbine wheel, an increase in the unbalance of the rotor occurs, as a result of which the bearing arrangement may also be damaged.
Therefore the nozzle rings and the moving blades of the turbine wheel must be regularly cleaned of the contaminants adhering to them.
DE-A1 35 15 825 discloses a method of and a device for cleaning the moving blades and the nozzle ring of the axial turbine of an exhaust-gas turbocharger having an inner bearing arrangement. The axial turbine has a gas-inlet casing having an outer and an inner casing wall, the latter serving to cover the turbine wheel and the shaft relative to the flow passage. The cleaning device comprises a plurality of water injectors arranged on the gas-inlet casing of the axial turbine and having nozzles, reaching into the flow passage, and a water line. At a certain degree of contamination of the axial turbine, a cleaning requirement is determined via a measuring and analyzing unit. Accordingly, water is injected into the flow passage via the nozzles arranged upstream of the guide vanes. The resulting water droplets are transported by the exhaust-gas flow up to the guide and moving blades respectively of the axial turbine and clean said blades of the adhering contaminants.
However, sufficient cleaning of the stationary guide vanes can only be achieved when the water droplets impinge as fully as possible on these guide vanes on their surface facing the exhaust-gas flow. To this end, the water injectors and the nozzles respectively must be arranged in a uniformly distributed manner over the entire periphery of the axial turbine. Accordingly, a larger number of injectors and nozzles are required, as a result of which such a solution becomes relatively complicated and thus expensive. In addition, the cost required to seal the gas-inlet casing increases with an increasing number of nozzles. A further problem is the arrangement of the nozzles in a region of the flow passage in which a relatively high flow velocity prevails. This results in a flat water jet, which only reaches parts of the guide vanes. Sufficient cleaning is therefore not ensured.